The present invention concerns a bicone or tricone bit drilling tool which is designed so as to allow for a suitable removal of cuttings of excavated earth between the conical drilling wheels as said earth cuttings form and are carried to the top of the well by drilling fluid. The present invention avoids any clogging of the tool by accumulating cuttings of the excavated earth.
The problem to be put forward, which the invention mainly concerns, is best illustrated by a particular case of a bicone bit, but this problem also exists as regards tricone and quadricone bits. For the following description, reference shall be made to the accompanying FIGS. 1 and 2 which respectively show an end view of a conventional bicone bit as currently known in the prior art, and a cutaway view along the line II--II of FIG. 1.
The bicone bit 10 comprises an approximately cylindrical steel body 12 provided at its upper end with a threaded portion 14 so as to connect the tool to a length of drill pipe or tubing which drives it in rotation. The bit 10 is provided at its other end with two diametrically opposite arms 16, 18, each of which extends approximately in an axial direction. Each arm 16, 18 bears a conical cutter wheel 24, 26 with several rows of teeth (not shown individually). Tubular projections 23, 25 are formed in the gaps defined between the arms 16, 18, said projections 23, 25 bearing at their ends nozzles 27, 29 through which a fluid is discharged. This fluid is directed by the nozzles 27, 29, either toward the formation so as to destroy it by means of impact, or toward the conical wheels 24, 26 so as to clean them. The arms 16, 18 and the tubular projections 23, 25 define a cavity 31 with an open bottom at the lower end of the body 12.
However, the drilling tools of this type, whether they be bicone or tricone bits, do have the drawback of being subject to clogging, known as bit balling. This is firstly owing to the fact that the cuttings of excavated earth naturally tend to accumulate in said hollow bottom cavity 31, and it is difficult for the excavated earth to rise along the wall of said cavity. The tendency toward clogging is secondly owing to the fact that there are only narrow passages between the arms 16, 18 and the projections 23, 25 for allowing the excavated earth cuttings and drilling fluid to flow to the outside of the tool 10. Cuttings flowing out of the cavity tend to flow back past the cutter wheels. Finally, there is a tendency toward clogging because the space available between the body 12 of the tool 10 and the wall of the drilled hole is extremely restricted. This restriction results from the fact that the bit body 12 is almost fully cylindrical, substantially matching the size and cylindrical shape of the drilled hole.
The removal of the excavated earth is thus slowed down and does not occur as quickly as these cuttings form. The clogging tendency of these tools is particularly significant when the formation is soft or sticky. The excavated earth then adheres to the walls of the cavity 31 and forms a compact hard mass which gradually fills the entire cavity 31, thus reducing the effectiveness of the wheels 24, 26. This also accordingly reduces the capacity of the drilling tool 10. This clogging tendency is promoted by the fact that the wall of the cavity 31 is connected to the outer cylindrical wall of the body 12, not by smooth transitions, but by relatively sharp edges which prevent the easy passage of this excavated earth.
U.S. Pat. No. 1,263,802 attempts to resolve this problem when using a bicone bit by suggesting securing to the tool a pair of plates which are shaped to form a shroud for directing fluid flow between the conical wheels and to reduce the volume of the cavity of the tool. However, this solution leads to a result contrary to the one sought after, since these plates also reduce the area of the gaps through which the excavated earth is removed. With this type of tool, the cavity is thus quickly clogged up.
For its part, the U.S. Pat. No. DE 1,223,779 suggests resolving the problem in question by fitting a bicone bit with a deflective helical plate for guiding the removal of the excavated earth outside the tool. But in practice, this plate, along with the two wheels and the body of the tool, forms a confined volume in which the excavated earth is trapped.
A further solution to the problem in question is contained in U.S. Pat. No. 4,285,409, which concerns a bicone bit obtained by omitting one of the wheels on one of the arms of a tricone bit and by placing on the freed arm a row of diamond insert studs and a row of nozzles. However, the trap for excavated earth cuttings is still present in this tool, as the gaps between the three arms of the tool are narrow, and they impede the removal of the excavated earth.